Waterflooding by regulating the total hardness within the drive water

ABSTRACT

Improved mobility control in waterflooding is obtained by following an aqueous solution containing a mobility reducing agent with water containing a minimum total hardness and thereafter increasing the total hardness of the drive water, preferably to a total hardness characteristic of water conveniently located to be used for the majority of the drive water. The total hardness of the water can be reduced by methods known in the art. Also, the total hardness can be increased, if necessary, by adding salts of Ca , Mg , etc. This invention is particularly useful with a miscible or miscible-like flooding agent followed by an aqueous mobility reducing agent (e.g. partially hydrolyzed, high molecular weight polyacrylamide) and this, in turn, followed by a minimum total hardness water and thereafter the total hardness of the water increased.

FIP'79125 OR 3 707 331) United States Patent a a 1151 3,707,;19 Davis,Jr. et al. 1 1 1 Dec. 26, 1972 [54] WATERFLOODING BY REGULATING PrimaryExaminer-Robert L. Wolfe THE TOTAL HARDNESS WITHIN THE Attorney-JosephC. Herring, Richard C. Willson, Jr. DRIVE WATER and Jack L. l-lummel[72] Inventors: John A. Davis, Jr., Littleton; John S. [57] ABSTRACTRhudy, Denver, both of Colo.

Improved mobility control in waterflooding is 0b- Assignee! Marathon onCompany Findlay tained by following an aqueous solution containing aOhio mobility reducing agent with water containing a [22] Filed; Dec.1970 minimum total hardness and thereafter increasing the total hardnessof the drive water, preferably to a total PP 97,690 hardnesscharacteristic of water conveniently located to be used for the majorityof the drive water. The total hardness of the water can be reduced bymethods I 22 known in the art. Also, the total hardness can be in- 58Field of Search 166/273 274 268 275 "eased, if mssary, by adding Salts 9y? etc. This invention is particularly useful with a miscible ormiscible-like flooding agent followed by an [56] References Citedaqueous mobility reducing agent (e.g. partially UNITED STATES PATENTShydrolyzed, high molecular weightpolyacrylamide) and this, in turn,followed by a minimum total hard- 3,352,358 Williams ness water andthereafter hardness of the 3,470,956 10/1969 Boston ..166/273 waterincreased;

3,428,l27 2/l969 Atkins ..l66/274 15 Claims, 1 Drawing FigurePERMEAE/L/TY REDUCTION A6 A FUNCTION OF TOTAL HARD/V555 0F D/SPLACl/VGWATER TOTAL HARDNESS, MG/LITER AS C000 PATENTED DEC 2 6 I972 NOLLOHGEHAJJ'IIBVBWHHd IA/VEA/TORS' JOHN A. DAVIS, JR. JOHN S. RHUDY A T TOR/V5 YWATERFLOODING BY REGULATING THE TOTAL HARDNESS WITHIN THE DRIVE WATERBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to an oil-bearing subterranean reservoir having an. injectionmeans in fluid communication with a production means and whereinawaterflood is carried out with a mobility reducing agent followed bydrive water. More particularly, a miscible or miscible-like displacingagent can precede the aqueous slug containing the mobility reducingagent.

2. Description of the Prior Art In flooding an oil-bearing subterraneanreservoir, mobility reducing agents are very useful toobtain bettermobility control of the waterflood and thus realize higher oilrecoveries. Examples of mobility control processes are taught in US.Pat. Nos. 3,406,754 to Gogarty; 3,443,635 to Gogarty et a1; 3,443,636 toGogarty; 3,467,187 to Gogarty et al; 3,497,006 to Jones et al; 3,506,070to Jones. Other patents which teach the use of particular mobilitycontrol agents in 2,843,545 to Wolf; 3,002,960 to Kolodny; 3,039,529 toMcKennon; 3,067,161 'to Roth; 3,282,337 to Pye; 3,370,647 to Wolgemuth,etc. In general, it is known that high molecular weight, water-solublepolymers which effectively increase the viscosity of water or reduce thepermeability to the flow of water are useful to obtain mobility control.Natural polymers such as those obtained from kelp weeds are also usefulas well as biopolymers.

Generally, the cost of the mobility reducing agent is instrumental indeciding whether or not to waterflood. Therefore, any reduction in thecost of the mobility reducing agent or any improvement or increase inefficiency of flooding is very important in deciding whether towaterflood a reservoir.

Applicants have discovered a novel process of reducing the concentrationand/or total amount of mobility reducing agent required to flood a givenoil-bearing reservoir.

SUMMARY OF THE INVENTION By following an aqueous slug containing amobility reducing agent with water containing a minimum totalDESCRIPTION OF THE DRAWING The drawing illustrates the relationship ofthe ratio of initial permeability to the flow of water (that is, beforeflooding a core with an aqueous solution containing a mobility reducingagent) divided by the final permeability (that is, the permeability tothe flow of drive water in the core after the mobility reducing agenthas been injected) vs. total hardness of the drive water flowing throughthe core. The total hardness is'in milligrams per liter and representsthe equivalent amount of magnesium and calcium cations within the waterconverted to calcium carbonate. Data for this curve is obtained byflooding a single core having an initial liquid permeability of 18] md.The mobility reducing agent is Pusher polymer 500 Series (Pusher is atrademark of Dow Chemical Company, Midland, Mich., and identifies apartially hydrolyzed, high molecular weight polyacrylamide). The core isfirst flooded with water A- (Table I) and is then flooded with 15 porevolumesof water A (Table I) containing 500 ppm. of Pusher polymer 500Series. Thereafter, there is injected consecutively equal volumes ofwaters B, C, and A,

identified in Table I. This curve illustrates that better mobilitycontrol is obtained by using a minimum total hardness water immediatelyafter the polymer slug.

DESCRIPTION OF PREFERRED EMBODIMENTS This invention is useful with a'waterflood wherein water containing a mobility reducing agent isinjected into an oil-bearing-subterranean formation and displaced towarda production means to recover crude oil through the production means. Inaddition, the aqueous solution containing the mobility reducing agentcan be preceded by a displacing slug, e.g. a miscible slug, amiscible-like slug, or any slug which effectively displaces crude oilfrom the reservoir. Examples of displacing slugs preferred with thisinvention include soluble oils (both anhydrous and hydrous solubleoils), micellar dispersions (both oil-external and water-external),emulsions, microemulsions, cosurfactant slugs such as alcohol (5) and/oraldehyde(s), hydrocarbon and/or water slugs containing alcohols,surfactants, etc. As mentioned earlier, any displacing slug which caneffectively displace crude oil from the reservoir can precede theaqueous slug containing the mobility reducing agent.

Examples of mobility reducing agents useful with the invention includehigh molecular weight natural polymers as well as synthetic polymers.Examples of these agents are taught within the patents enumeratedearlier under the paragraph Description of the Prior Art. Preferredexamples of mobility reducing agents include high molecular weightpolyelectrolytes, specifically high molecular weight polyacrylamides andpartially hydrolyzed products thereof, sulfonated high molecular weightpolymers, chemically modified natural polymers such as CMC (carboxylmethyl cellulose), and like polymers. A particularly preferred mobilityreducing agent is the Pusher polymer sold by Dow Chemical Company,Midland, Mich. The polyelectrolyt'es should have an average molecularweight above about 250,000, preferably above 500,000 and more preferablyabout 1 to about 10 million.

The volume of aqueous mobility reducing agent injected into thereservoir can vary dependent upon the conditions of the reservoir, theviscosity of the crude oil, etc. The concentration of the mobilityreducing agent within the aqueous slug also depends upon the viscosityof the crude oil, interstitial water, combination of the two, and moreparticularly the mobility of v the formation fluids (i.e. crude oil plusinterstitial water within the reservoir). Examples of preferred designsof the aqueous mobility reducing agent slug and the mobility thereof aretaught within patents listed under Description of the Prior Art.

The aqueous solution containing the mobility reducing agent is followedby water containing a minimum total hardness. Such reduces theunfavorable mobility contrast that exists between the aqueous slugcontaining the mobility reducing agent and the drive water. During thelatter stages of drive water injection, the total hardness is increasedto increase the effective permeability to the flow of the drive water..Total hardness is defined as'the equivalent molecular amount of calciumand magnesium converted to calcium carbonate. Preferably, the aqueousslug containing the mobility reducing agent is -followed by up to about25 percent or more and more preferably up to percent formation porevolume of the minimum total hardness water. Thereafter, the totalhardness of the drive water is increased to a maximum value thatpreferably is characteristic of the water that is most economicallyavailable to inject into the reservoir. More preferably, the water isgradually graded from a minimum total hardness value up to a totalhardness characteristic of the water readily available. From theattached FIGURE, it is readily apparent that water having an increasedtotal hardness has an effect that increases the relative effectivepermeability to the flow of that water within the reservoir. Therefore,by increasing the total hardness of the water, the project life can bereduced since the effective permeability to the drive water is increaseddue to the total hardness as illustrated in the FIGURE, and thus theinjection wells can receive a higher injection rate of water. Totalhardness can be increased, when needed, by adding Ca and Mg salts to thewater. Examples of a preferred minimum total hardness of a drive wateris less than about 100 milligrams per liter and an example of a highertotal hardness of a drive water is greater than about 200 milligrams perliter.

The water readily available at the watertlooding project can have itstotal hardness reduced by methods known in the art. A very common methodis 'to treat the water with a zeolite resin which exchanges sodiumcation for the magnesium and calcium cations. However, any ion exchangeresin or method similar thereto or any method which effectively removescalcium and magnesium cations from the water is useful with thisinvention.

It is understood that additives can be incorporated within the drivewater. Such additives include corrosion inhibiting agents, bactericides,etc.

EXAMPLE The following example is presented to teach specific embodimentsof the invention. Unless otherwise specified, all percents are based onvolume:

' A sandstone core taken from a reservoir in Eastern lllinois, U.S.A. iscleaned and dried. The core has an in- (him the core. After the polymeris injected, three different drive waters are injected into the core andthe effective permeability to the flow of the water in each of thesecores is measured. It is noted that the permeability of the core priorto polymer injection is not sensitive to ion content-in the water.Analysis of the injected waters is given in Table l.

The measured permeability of the core to each of the three waters alongwith the permeability reduction values (i.e. initial water permeabilitydivided by the flushed permeability or the permeability to the flow of aparticular water) are given Table 11.

TABLE II PERMEABILITY OF THE CORE Flushed Water PermeabilityPermeability (md) Reduction By inspecting Tables I and II, it is readilyapparent that the permeability reduction is not directly dependent uponthe total dissolved salt level in the drive water. Rather, the effectivepermeability reduction is directly dependent upon the total hardnesslevel of the drive water as illustrated in Table 11. That is, for adrive water having a total hardness of 72, the effective permeabilityreduction obtained is 15.4 whereas for water C having a total hardnessof 430, the effective permeability reduction is l 1.8 and for water Bhaving a total hardness of 760, the effective permeability reduction is7.0.

Using data similar to that in Table 11 and specifically the attachedFIGURE, improved mobility control can be designed into waterfloodingprojects. That is, the initial drive water following the aqueous slugcontaining the mobility reducing agent should contain a low totalhardness to minimize the unfavorable mobility contrast that existsbetween the aqueous slug containing the mobility reducing agent and thedrive water. During the latter stages of drive water injection, thetotal hardness can be increased to increase the effective permeabilityto the flow of the drive water and thus reduce the life of the floodingproject and permit an earlier return on invested capital. Such allowsfor an increased injection rate which means reduced project life. Thus,this invention can be used to decrease the unfavorable mobility contrastbetween the aqueous slug containing the mobility reducing agent andinitial drive water and thus it is possible to decrease the size of thepolymer slug 1. An improved process of waterflooding an oil-bear ingsubterranean reservoir having an injection means in fluid communicationwith a production means, the process comprising injecting into thereservoir an aqueous slug containing a. mobility reducing agent andthereafter injecting drive water containing a minimum total hardness andfollowing this drive water with drive water containing an increasedtotal hardness, relative to the minimum total hardness, to displacecrude oil toward the production means.

2. The process of claim 1 wherein a miscible slug or a miscible-likeslug precedes the injection of the aqueous solution containing themobility reducing agent.

3. The process of claim 1 wherein a micellar dispersion precedes theaqueous slug containing the mobility reducing agent.

4. The process of claim 1 wherein a soluble oil precedes the aqueousslug containing the mobility reducing agent.

5. The process of claim 1 wherein up to about 25 percent formation porevolume of the minimum total hardness drive water is injected andthereafter there is injected water containing a relatively higher totalhardness.

6. The process of claim 1 wherein the mobility reducing agent is a highmolecular weight polyelectrolyte.

7. An improved process of recovering crude oil from an oil-bearingsubterranean formation having an injection means in fluid communicationwith a production means, the process comprising:

' l. injecting a miscible or miscible-like displacement slug into thereservoir,

2. injecting into the reservoir an aqueous solution containing amobility reducing agent,

3. injecting up to 25 percent formation pore volume of a drive watercontaining a minimum total hardness and I 4. thereafter injecting drivewater containing a relatively higher total hardness, as compared to theminimum total hardness, the overall effect being to impart a moreefficient mobility control to the waterflooding process.

8. The process of claim 7 wherein the displacing fluid is a micellardispersion.

9. The process of claim 7 wherein up to about 50% formation pore volumeof the aqueous solution contains the mobility reducing agent is injectedinto the reservoir.

10. The process of claim 7 wherein the mobility reducing agent is a highmolecular weight polyelectrolyte.

11. The process of claim 10 wherein the mobility reducing agent is apartially hydrolyzed, high molecular weight polyacrylamide. I

12. The process of claim 7 wherein the total hardness of the drive wateris gradually increased from the minimum total hardness to the relativelyhigher total.

hardness.

13. The process of claim 7 wherein the relatively higher total hardnessin the drive water is obtained by adding salts of Ca and/or Mg to thedrive water.

14. The process of claim 7 wherein the minimum total hardness of thedrive water in 3. is less than about milligrams per liter.

15. The process of claim 7 wherein the relatively higher total hardnessof drive water in 4. is greater than about 200 milligrams per liter.

2. injecting into the reservoir an aqueous solution containing amobility reducing agent,
 2. The process of claim 1 wherein a miscibleslug or a miscible-like slug precedes the injection of the aqueoussolution containing the mobility reducing agent.
 3. injecting up to 25percent formation pore volume of a drive water containing a minimumtotal hardness and
 3. The process of claim 1 wherein a micellardispersion precedes the aqueous slug containing the mobility reducingagent.
 4. The process of claim 1 wherein a soluble oil precedes theaqueous slug containing the mobility reducing agent.
 4. thereafterinjecting drive water containing a relatively higher total hardness, ascompared to the minimum total hardness, the overall effect being toimpart a more efficient mobility control to the waterflooding process.5. The process of claim 1 wherein up to about 25 percent formation porevolume of the minimum total hardness drive water is injected andthereafter there is injected water containing a relatively higher totalhardnesS.
 6. The process of claim 1 wherein the mobility reducing agentis a high molecular weight polyelectrolyte.
 7. An improved process ofrecovering crude oil from an oil-bearing subterranean formation havingan injection means in fluid communication with a production means, theprocess comprising:
 8. The process of claim 7 wherein the displacingfluid is a micellar dispersion.
 9. The process of claim 7 wherein up toabout 50% formation pore volume of the aqueous solution contains themobility reducing agent is injected into the reservoir.
 10. The processof claim 7 wherein the mobility reducing agent is a high molecularweight polyelectrolyte.
 11. The process of claim 10 wherein the mobilityreducing agent is a partially hydrolyzed, high molecular weightpolyacrylamide.
 12. The process of claim 7 wherein the total hardness ofthe drive water is gradually increased from the minimum total hardnessto the relatively higher total hardness.
 13. The process of claim 7wherein the relatively higher total hardness in the drive water isobtained by adding salts of Ca and/or Mg to the drive water.
 14. Theprocess of claim 7 wherein the minimum total hardness of the drive waterin
 3. is less than about 100 milligrams per liter.
 15. The process ofclaim 7 wherein the relatively higher total hardness of drive water in4. is greater than about 200 milligrams per liter.